Frequency synthesizers are used extensively in modern portable and mobile communications equipment and their performance is a key factor in interference reduction. One factor to be considered when using these devices is the battery voltage range. When designing a radio with a frequency synthesizer for use in autos or in a base station with battery backup system, the radio design most often requires the frequency synthesizer voltage supply regulation be set at 9.5 volts or lower. This is to insure the regulator stays out of saturation under all operating conditions. When this limitation is combined with the voltage saturation limits of the voltage controlled oscillator steering circuits, in the synthesizer phase locked loop, the maximum steering range of the voltage controlled oscillator (VCO) is reduced to about 8.5 volts DC or lower. This limited voltage is a problem because for better VCO performance it is desirable to increase this voltage to approximately 11 volts DC.
Normally, this is accomplished by running the phase locked loop charge pump off a higher voltage source such as a switching supply of approximately 12 volts. The problem associated with this technique is that the switching supply generates large amounts of electrical noise. Even if only microvolt spurs of electrical noise are produced, this noise is nevertheless coupled to the VCO and performance will greatly be reduced.
Therefore, the need exists for an electrical circuit which will allow steering of a charge pump current source within a phase locked loop to a higher voltage without utilizing a switching supply. Only in this way can switching noise and other electromagnetic interference (EMI) be eliminated so as not to degrade radio performance.